486 STRUCTURE OF SHELLS OF FORAMINIFERA. 



in the study of the Foraminifera. For it has come to be generally 

 admitted that ' plan of growth ' is a character of very subordinate 

 importance among the Foraminifera, so that any classification which 

 is primarily based upon it must necessarily be altogether un- 

 natural ; those characters being of primary importance which have 

 an immediate and direct relation to the Physiological condition of 

 the Animal, and are thus indicative of the real affinities of the 

 several groups which they serve to distinguish. The most impor- 

 tant of these characters will now be noticed.* 



369. Two very distinct types of Shell-structure prevail among 

 ordinary Foraminifera, — namely, the Porcellanous, and the Hya- 

 line or Vitreous. In the former, the shell when viewed by reflected 

 light presents an opaque-white aspect which bears a strong resem- 

 blance to Porcelain ; but when thin natural or artificial lamina? of 

 it are viewed by transmitted light, the opacity gives place to a 

 rich brown or amber colour, which in a few instances is tinged with 

 crimson. No structure of any description can be detected in this 

 kind of Shell-substance, which is apparently homogeneous through- 

 out. Although the shells of this ' Porcellanous ' type often present 

 the appearance of being perforated with Foramina, yet this appear- 

 ance is illusory, being due to a mere ' pitting ' of the external 

 surface, which, though often very deep, never extends through 

 the whole thickness of the shell. Some kind of inequality of that 

 surface, indeed, is extremely common in the Shells of the porcel- 

 lanous Foraminifera ; one of the most frequent forms of it being 

 a regular alternation of ridges and furrows, such as is occasionally 

 seen in Miliola (Plate xv., fig. 3), but which is an almost constant 

 characteristic of Peneroplis (fig. 5). But no difference of texture 

 accompanies either this or any other kind of inequality of surface ; 

 the raised and depressed portions being alike homogeneous. — In the 

 shells of the Vitreous or Hyaline type, on the other hand, the proper 

 Shell-substance has an almost glassy transparence, which is shown 

 by it alike in thin natural lamella?, and in artificially-prepared 

 specimens of such as are thicker and older. It is usually colour- 

 less, even when (as is the case with many Rotalina?) the substance 

 of the Animal is deeply coloured ; but in certain aberrant Rotalines 

 the Shell is commonly, like the animal body, of a rich crimson hue. 

 All the Shells of this type are beset more or less closely with 

 tubular perforations, which pass directly, and (in general) without 

 any subdivision, from one surface to the other. These Tubuli are 

 in some instances sufficiently coarse for their orifices to be distin- 



* This subject will be found amply discussed in the Author's " Intro- 

 duction to the study of the Foraminifera," published by the Ray Society ; 

 to which work he woidd refer such of his readers as may desire more 

 detailed information in regard to it. It has been with great satisfaction 

 that he has found his own views on this subject to be in full accordance 

 with those of Prof. Reuss, of Vienna, the highest Continental authority 

 upon this group. 



